In the field of audio enabled devices, for example wireless communication devices such as mobile telephone handsets, an audio output typically requires a low impedance speaker and a performance audio amplifier. In the context of a wireless communication device, these elements are selected to achieve high power efficiency in order to minimise power consumption, and thereby improve battery life. Class-D amplifiers, which use switching modes of transistors to regulate power delivery, are good candidates, and thus are typically used for such audio applications.
FIG. 1 illustrates an example of a conventional Sigma-Delta based class-D amplifier 100. The amplifier 100 comprises PDM (Pulse Density Modulation) Sigma-Delta modulator circuitry 110, operably coupled to a low distortion power driver 120, which in turn is operably coupled to a speaker 130. Such a conventional amplifier arrangement comprises good total harmonic distortion and noise (THD+N) characteristics, a good power supply rejection ratio (PSSR), and good immunity to electromagnetic interference (EMI).
FIG. 2 illustrates a graphical example 200 of the output power spectrum against frequency for the amplifier 100 of FIG. 1. As can be seen, at lower audio frequencies, the impedance 210 of speaker 130 is low. Accordingly, the majority of the power spectrum at these frequencies comprises the active power (audio signal). However, the speaker 130, due to its inductance model, naturally performs low-pass filtering of the received signal in the analogue domain and removes part of the quantization noise and signal image. Accordingly, at higher frequencies, the impedance of the speaker 130 increases. In particular, at frequencies corresponding to the shaped quantization noise and signal image, the impedance of the speaker 130 is significant, resulting in significant power attenuation at these higher frequencies.
The power efficiency of amplifier 100 may be defined as:
  Efficiency  =                    Vout        audio        2                    Zspk        audio                                                                                Vout                audio                2                                            Zspk                audio                                      +                                          Vout                HF                2                                            Zspk                HF                                      +                                                            (                                                            Vout                      audio                                                              Zspk                      audio                                                        )                                2                            ·                                                                        Ron            +                                          Vbat                2                            ·                              C                par                            ·                              f                s                                      +                          Vaudio              ·              Icc                                          where:
      Vout    audio    2        Zspk    audio  represents the active power (audio signal);
      Vout    HF    2        Zspk    HF  represents the high frequency (HF) power attenuation (sigma-delta modulation and signal image);
            (                        Vout          audio                          Zspk          audio                    )        2    .represents the conductive power loss (ON resistance of power stage);
Vbat2·Cpar·f represents the switching power loss; and
Vaudio·Icc represents the bias power loss (class-D internal consumption).
The higher frequency power attenuation is particularly significant for low level input signals, where the quantization noise becomes proportionately large. Consequently, the power efficiency of the conventional amplifier circuitry 100 of FIG. 1 for low level input signals is dependent, to a large extent, on the speaker impedance at these higher frequencies. Accordingly, the speaker impedance characteristics are a considerable contributor to power efficiency loss of the amplifier circuitry 100, and as such the power efficiency loss can vary from speaker to speaker. The kind of audio architecture illustrated in FIG. 1 may require an external LC filter to improve the power efficiency during low level input signals. Consequently, the choice of speakers that might be suitable for use within an audio enabled device using such a conventional amplifier is limited by the impedance of the speakers, irrespective of any other beneficial or desirable features that the speakers may have.